TRACON Controller reported confusion between similar call signs as aircraft entered a higher MVA which resulted in a CFTT encounter and failure to issue a low altitude alert.

Narrative

I was working Final in WIDE SIMUL (simultaneous arrival sequencing) configuration. I was almost consistently getting the visual approach around 100 at ZZZZZ. Because of this; Sector X did not abide by the feed rule for SIMULs and took Aircraft Y direct ZZZZZ descending to 100. I called to ask them for the airport in sight and did not get a response. (Upon listening to radar display replay; it is because I called them Aircraft X (another Aircraft X that just checked on frequency in the downwind). At this point; Aircraft Y is getting very close to Aircraft Z that Sector X is working to Runway XX without ensured separation. I issue what I thought was Aircraft Y (turns out I said Aircraft X again) an expeditious descent to 080 and asked again for the airport. They (Aircraft X) called the airport in sight. I cleared Aircraft Y for the visual approach with a speed (they had not called the airport in sight; Aircraft X in the downwind did) and Aircraft Y read back the visual approach and speed. I did not realize I had mistakenly transposed callsigns. A few moments later in my scan I realized Aircraft X was at 080 in a 085 MVA and I quickly climbed them to 085 (I did not issue a low altitude alert as I know VFR aircraft are in that exact spot every day at 055.)Recommendation: I completely messed this up. In review of radar display replay I definitely said the instructions to the incorrect callsign. I'm surprised Aircraft Y read back the visual approach clearance when I hadn't asked for the airport and that Aircraft X didn't query me when I cleared a different aircraft for the visual after asking them; but this is 100% on me. I haven't had a situation like this in years; it is so easy to happen and happens in unfortunately really bad situations a lot. I wish the feeder would have done their job and ensured separation so I didn't need to issue an expeditious descent; but I am solely the one that used the incorrect callsign. This is why I as a feeder have started using crossing restrictions to ensure separation. I will 100% pay closer attention to similar sounding callsigns; I didn't even notice this was an issue until I realized what had happened. As for changes; had this been fed to me properly I wouldn't have needed to use the expedite; but I can't say I wouldn't have messed up the callsigns still. I need to be more careful and that's really all I can say.